Normal maps only lights up one side, while the other one stays dark with an all directional light source
Try changing Object space to tangent space in the normal map node.

Object Space Normals are likely to cause unexpected shading effects, as they do not support object transformation and mesh deformations, like tangent space normals do.
Most of the time normal maps need to be applied relative to the surface of the mesh, not to the whole object.
(https://knowledge.autodesk.com/support/maya-lt/learn-explore/caas/CloudHelp/cloudhelp/2015/ENU/MayaLT/files/Asts-Object-space-world-space-and-tangent-space-htm.html)